1. Field of the Invention
The present invention relates to direct radiography. The invention more particularly relates to a method for correcting defective pixel image artifacts in such a way that the visibility of reconstructed defective pixel clusters located in image-regions with strong signal-gradients is significantly reduced.
2. Description of the Related Art
Static and dynamic flat panel detector based x-ray imaging systems are commonly used in various application areas ranging from non-destructive testing to medical diagnosis. Although these complex, solid state sensor based image-acquisition devices are calibrated on a regular basis to determine and compensate for the various sources of spread in the signal conversion characteristics of their sensor-pixels, array-wise arranged in the detector's sensitive entry-surface, some of these sensor-pixels are defective or behave irregular causing their image-data to be non-representative for the amount of light or x-rays they have been exposed to during signal integration.
The majority of these unreliable sensor-pixels, often referred to as defective pixels generating invalid image-data, are isolated pixels which are distributed across the surface of the array-sensor.
Direct neighbor kernel based reconstruction algorithms making use of the eight non-defective image-data immediately surrounding the isolated defective pixel are sufficient to calculate a very effective replacement value for the defective pixel.
The defective pixel becomes well hidden after reconstruction and its uncorrected image-artifact, the local image-impact, disappears nearly completely.
Even when the reconstruction value of that isolated pixel might slightly differ from its normal value, the image data generated at that specific image-location in case the sensor-pixel wouldn't have been defective, a small reconstruction error, the deviation between the replacement value and the normal value, still remains hardly detectable in the corrected image.
Depending on the nature and the type of the physical phenomenon which is responsible for the invalid or unstable exposure-response of a defective pixel, groups of clustered defective pixels with various shapes and spatial extent are inevitably generated too. The image-impact of defective clusters is larger since multiple closely grouped image-pixels are affected is a small image-region.
Even when a reconstruction value is calculated for each individual defective pixel in the cluster, the reconstruction requirements shouldn't be less severe than those applicable for the reconstruction of an isolated defective pixel because a group of clustered pixels exhibiting small reconstruction errors will more likely be regarded as an artificial and thus disturbing, visible image-artifact, revealing the cluster's inadequately hidden presence and size.
This phenomenon becomes even worse if the defective cluster is surrounded by image-data showing a signal-slope profile due to the presence of a local signal-gradient often induced by to the sudden, step-shaped contrast differences at the boundaries of radiographed objects with substantially different x-ray absorption properties like bone and soft tissue.
Due to the grouped presence of other defective pixels in the immediate neighborhood of one of the defective pixels under reconstruction, the residual direct neighbor kernel or the side-wise extended kernel from which the replacement value is calculated loses geometrical balance relative to its reconstruction point and is drawn towards a spatial region with on average higher or lower image-data.
As a result the replacement value obtained for the defective pixel will generate a larger reconstruction error producing a local dented disturbance in the often fairly straight iso-contour lines thus making it even more difficult to hide the defective pixel cluster.